1. Field of the Invention The present invention relates to embedded prisms, and more particularly concerns correction of lateral chromatic aberration of light transmitted through such an embedded prism.
2. Description of Related Art
A type of beamsplitter widely used for selectively passing and reflecting a light beam includes a thin, flat, parallel sided, transparent plate mounted in a transparent liquid or solid medium at an angle, commonly about 45.degree., to the axis of a beam of light that is to be transmitted or reflected. Such beamsplitters may be polarizing or nonpolarizing. A polarizing beamsplitter, but not of the embedded type, is described in U.S. Pat. No. 2,403,731 issued to MacNeille. The polarizing beamsplitter, such as the MacNeille type polarizing beamsplitter will pass light having one polarization state, such as the "P" state for example, and reflect light with another polarization state, such as the "S" state for example. Thus, the polarizing beamsplitter selectively passes or transmits a light beam, depending upon whether the polarization vector of the light is one or the other of two mutually orthogonal directions. In the beamsplitter described in the MacNeille patent, a plurality of dielectric layers of appropriate indices of refraction and thicknesses are deposited at the interface between the two halves of a glass cube of which the mating interface extends diagonally between two diagonally opposite edges of the cube.
In an embedded MacNeille polarizing beamsplitter, a housing of generally cubic configuration is provided with transparent front, back, entrance and exit windows. This is filled with a fluid in which is suspended a prism plate comprising a thin plate with mutually parallel planar sides that extends diagonally across the cube. A plurality of thin dielectric layers, of the type described in the MacNeille patent, may be applied to the thin plate to make this embedded prism a MacNeille polarizing prism. Such embedded prisms exhibit a color defect known as lateral chromatic aberration, which significantly decreases clarity and resolution of transmitted light and also significantly reduces contrast, thereby producing an image of decreased quality. This aberration is due to the different variation of index of refraction with color from one material to another, as will be explained below.
To avoid bending of the light transmitted through the embedded prism plate, the prism and the fluid in which it is immersed are made of materials selected to have matching indices of refraction. As is well known, the index of refraction of a material is proportional to the reciprocal of the velocity of light propagated in the material, and such velocity varies from one material to another. Thus, as the light passes from one material to another with a different index of refraction, the light beam is bent. Accordingly, an embedded prism must be constructed with materials having the same index of refraction insofar as possible, if beam bending is not desired. However, the index of refraction of any given material varies non-linearly with the wavelength of the light or its color. Accordingly, when matching indices of refraction of two materials, it is common to employ the peak visual wavelength, green, which has a wavelength of about 550 nanometers (550.times.10.sup.-9 meters or 0.55 micrometers, which is approximately 21.7 microinches) when comparing indices of refraction. Nevertheless there exists for each material an unique, non-linear color sensitive variation (from one wavelength to another) of its index of refraction. The non-linear variation of index of refraction with color (wavelength) itself varies from one material to another, even if the indices of the two materials are the same at some given wavelength. Therefore, although two materials may be selected to have nominally equal indices of refraction, in actual fact the indices of refraction are equal only at the particular "match" wavelength.
Refraction of a component of light at an interface between two media depends upon both the difference in indices of refraction of the media and the angle of incidence of the light upon the interface. When a converging or diverging beam of light is directed at the reflection/transmission interface, generally inclined at 45.degree. to the axis of the incident beam in a beamsplitter, the angle of incidence of different rays of the same beam on the interface varies as the distance of the point of impingement of the ray from the center line of the light beam increases.
The described lateral chromatic aberration is a color distortion that is not uniform over the field of the light beam, but generally increases with increasing distance from the optical axis. At any given distance from the optical axis, in any direction, the magnitude of this chromatic aberration is generally the same.
Past efforts to minimize adverse effects of such lateral chromatic aberration of embedded prisms have generally been directed toward selection of materials having indices of refraction that not only match at a given wavelength, such as 550 nanometers, but in which, for one material, the non-linear variation of the refracted index with color is either of a minimal value or also closely matches the non-linear variation of index of refraction of the other material. Complete and satisfactory avoidance of this problem has not been achieved in the past, even with the use of very expensive materials, such as fused silica for the prism plate and a specifically manufactured fluid, such as Cargille standard fused silica matching liquid, code 50350.
Although the embedded prism polarizing beamsplitter, e.g. an embedded MacNeille type polarizing beamsplitter, is useful in a wide variety of applications, one example of such an application is a color projection system employing a liquid crystal light valve. Examples of such projection systems are described in U.S. Pat. No. 4,343,535 to Bleha, Jr. and in U.S. Pat. No. 4,650,286 to Koda et al. A color projection system of this type is made and sold by Hughes Aircraft Company as HDP-6000 Model 700. The model 700 series of this projection system uses an embedded prism polarizing beamsplitter having a fused silica T12 OPTOSIL prism plate having an index of refraction of 1.459723 at 554.5 nanometers suspended in the Cargille code 50350 fluid. Hughes Aircraft Company HDP-6000 color projection system Models 800 and 1000 use a prism plate of Schott BK7 glass having an index of refraction of 1.518298 at 554.5 nanometers and a Cargille code 1160 fluid having an index of refraction of 1.517903 at the same wavelength. In such a color projection system, as described in detail in these patents, light from a light source is reflected from a MacNeille prism to a liquid crystal light valve which causes the light valve to retroreflect light of a particular polarization in accordance with modulation imposed on the light valve by an image generator, such as a cathode ray tube. The uniquely polarized light modulated and retroreflected from the light valve is then transmitted through the embedded MacNeille prism and projected via a projection lens. Transmission of the polarized light through the embedded MacNeille prism is subject to the above-described color aberration, despite the selection of expensive materials for the embedded MacNeille prism, whereby contrast and resolution of the resulting projected image is significantly degraded.
Accordingly, it is an object of the present invention to provide color correction for an embedded prism.